Final answer:
The position of a 4) pilot aboard a steamboat paid the most, as they were responsible for navigating the vessel safely, a demanding and critical task.
Step-by-step explanation:
According to the descriptions provided in various texts about steamboats during the time of Mark Twain, the position of a 4) pilot aboard a steamboat paid the most.
Steamboat pilots were highly skilled professionals responsible for navigating the vessel safely, which was a critical task especially on busy or challenging rivers like the Mississippi.
Twain himself worked as a steamboat pilot before becoming a famous writer, and his experience no doubt provided him with insight into the intricacies and the risks associated with navigating steamboats, thus highlighting the importance and value of the pilot's role.